Need Help Troubleshooting Mini Cooper Battery

[Lotsokids]

I know there are some electronically gifted members here. So I'll throw this question out there.

I own a 1990 Mini Cooper. My wife called today at 12:00 and said she parked the car and when she returned the battery was dead. I came and looked at it. Dead as a hammer. No headlights, dash lights... nothing. I tried to jump it from my [running] car, but still absolutely nothing! All fuses were good. I left the car and returned again 3 hours later with a meter. I checked the battery... 12 volts. The ground through the chassis was perfect. Then I turned on the headlights and they worked! I put the key in and it started and ran perfectly.

When I brought it home, I removed the battery and cleaned the connections. They did not look corroded. The green dot was visible on the top of the battery (showing status is good). Everything still operates perfectly. I don't have a lot of faith this thing won't leave my wife stranded again.

I'm at a loss. Anyone have any ideas? Thanks in advance!

[movenon]

 How old is the battery?  Perhaps a cell connection is opening intermittently ? 
George

[red]

Lotsokids,

You might have nailed it with cleaning the battery terminals, even if they did not look corroded.  A tip for next time: take a RESISTANCE reading from the center of the battery terminal to the outside of the terminal clamp.  You want no resistance there (100 ohms or less, maybe).  Do the same for each terminal.  If you get high resistances there, the terminals are corroded, no matter how they may look.

At the risk of being obvious, battery cables (large and small) have two ends each.  Check for a corroded connection where the negative cable bolts to the chassis ground, and where the positive cable bolts into the electrical system.  Without knowing Mini-Coopers, you may have a small positive cable to power the car, and a large positive cable to power the starter.  Both of those cables would need to be checked, if you have two positive cables.

Check that the cables do not appear swollen in any one place, especially at the terminal ends.  That swelling would mean that internal cable corrosion (not the terminals) will be at fault.  You can check the terminal ends for reliability by turning on the headlights, and then bending the cables slightly when the terminal ends are bolted securely.  If the lights go out then, you have your hand on a bad cable.  You might want to replace both positive and negative cables then, anyway, since high reliability is the goal here. 

Beyond this point, things can get complicated.

Cheers,
Red

[Lotsokids]

How old is the battery?

This battery was in the car when I bought it less than a year ago. It appears new, but I cannot find any receipt or documentation.

Red - The cables appear to be in good condition. I had my meter in the "diode test" position and it went to zero (no resistance) from the negative battery post to the trunk hinge pin.

Thanks for the ideas!

[red]

How old is the battery?

This battery was in the car when I bought it less than a year ago. It appears new, but I cannot find any receipt or documentation.
Red - The cables appear to be in good condition. I had my meter in the "diode test" position and it went to zero (no resistance) from the negative battery post to the trunk hinge pin.
Thanks for the ideas!

Lotsokids,

There are several ways that test may be inconclusive.  If you want reliability, start with some tools, and my last post.  You don't even need tools, for the cable-bending test.  I think it is safe to say, there is a real problem somewhere with that car, and it probably won't fix itself.  Forget any cheap fixes, okay?  Just replace the battery, and all of the cables connected to the battery.  Leaving your lady unsafe and stranded is not an option.
Red

[Dogsbestfriend]

Good calls about the battery cables. I once had an Astra diesel van that would randomly lose all electrics while being driven. I had great fun frantically waving out of the window while trying to coast out of traffic. A fitter and I finally decided that the only place the fault could be was where the cable was swaged into the battery terminal even though it looked fine. Cutting off the terminal and fitting a new one did the job. In the old days we the lead cups that fitted completely over the battery terminals and were held down with a single screw down through the top. Anyway they were notorious for losing connectivity through corrosion. I just used to melt them off the cable with a blow torch.
Those old Minis were great fun to drive especially when things got slippery.

[red]

Good calls about the battery cables. I once had an Astra diesel van that would randomly lose all electrics while being driven. I had great fun frantically waving out of the window while trying to coast out of traffic. A fitter and I finally decided that the only place the fault could be was where the cable was swaged into the battery terminal even though it looked fine. Cutting off the terminal and fitting a new one did the job.

Dogsbestfriend,

We did some cable testing for some jet engine guys once, and the engines had a three-wire cable bundle, using wires that would be good for automotive jumper cables.  These cables had massive gold-plated connector pins at the ends, swaged onto each wire with a large hydraulic press.  One cable failed repeatedly, not in resistance, but in impedance.  Those guys were shocked that a brand-new cable on their engines could fail like that, when their test gear found no problem.
Happy to help!    
Red

[fj1289]

Could it be in the ignition switch?

Do any mini forums mention this a a common issue?

[red]

Could it be in the ignition switch? Do any mini forums mention this a a common issue?

FJ1289,

I would expect headlights to be independent of the ignition system, and the OP said "no headlights." 
Without the Mini-Cooper schematics, though, I'm shooting in the dark here.

Red

[Lotsokids]

Could it be in the ignition switch? Do any mini forums mention this a a common issue?

FJ1289,

I would expect headlights to be independent of the ignition system, and the OP said "no headlights." 
Without the Mini-Cooper schematics, though, I'm shooting in the dark here.

Red

That's correct.
Again, thanks for the ideas, guys.